Image processing apparatus

A non-transitory computer-readable storage medium having an image processing program stored thereon and readable by a computer, the image processing program, when executed by the computer, causes the computer to implement functions comprising:

• a reduction function of executing color-number reduction processing for each block configured by a plurality of pixels included in a processing target image expressed by processing target image data, the color-number reduction processing comprising reducing the number of colors expressed by the plurality of pixels in the block to generate image data having the reduced number of colors from the processing target image data, a gradation-number of each color value included in the image data having the reduced number of colors being the same as a gradation-number of each color value included in the processing target image data;
• and
• a compression function of executing compression processing using the image data having the reduced number of colors to thus generate compressed image data,
• wherein the reduction function executes first color-number reduction processing by using a first type of the block constituted by N1 pixels, N1 being an integer of 2 or larger,
• wherein when a data size of the compressed image data after the first color-number reduction processing is larger than a threshold value, the reduction function executes second color-number reduction processing by using a second type of the block constituted by N2 pixels, N2 being an integer larger than N1, and
• wherein the processing target image data used in the second color-number reduction processing is the processing target image data before the first color-number reduction processing.

An image processing apparatus comprising:

• a processor configured to operate as:
• a reduction unit configured to execute color-number reduction processing for each block configured by a plurality of pixels included in a processing target image expressed by processing target image data, the color-number reduction processing comprising reducing the number of colors expressed by the plurality of pixels in the block to generate image data having the reduced number of colors from the processing target image data, a gradation-number of each color value included in the image data having the reduced number of colors being the same as a gradation-number of each color value included in the processing target image data;
• and
• a compression unit configured to execute compression processing using the image data having the reduced number of colors to generate compressed image data,
• wherein when a difference value indicating a difference of colors between two pixels of the plurality of pixels included in the block before color-number reduction processing is within a standard range, the color-number reduction processing comprises processing of setting color values of the two pixels to be the same,
• wherein when the difference value between the two pixels before color-number reduction processing is within a first standard range, the reduction unit is configured to execute first color-number reduction processing by executing the processing of setting color values of the two pixels to be the same,
• wherein when a data size of the compressed image data after the first color-number reduction processing is larger than a threshold value, and when the difference value between the two pixels before color-number reduction processing is within a second standard range, which is wider than the first standard range, the reduction unit is configured to execute second color-number reduction processing by executing the processing of setting color values of the two pixels to be the same, and
• wherein the processing target image data used in the second color-number reduction processing is the processing target image data before the first color-number reduction processing.

A non-transitory computer-readable storage medium having an image processing program stored thereon and readable by a computer, the image processing program, when executed by the computer, causes the computer to implement functions comprising:

• a reduction function of executing color-number reduction processing for each block configured by a plurality of pixels included in a processing target image expressed by processing target image data, the color-number reduction processing comprising reducing the number of colors expressed by the plurality of pixels in the block to generate image data having the reduced number of colors from the processing target image data, a gradation-number of each color value included in the image data having the reduced number of colors being the same as a gradation-number of each color value included in the processing target image data;
• and
• a compression function of executing compression processing using the image data having the reduced number of colors to thus generate compressed image data,
• wherein when a difference value indicating a difference of colors between two pixels of the plurality of pixels included in the block before color-number reduction processing is within a standard range, the color-number reduction processing comprises processing of setting color values of the two pixels to be the same,
• wherein when the difference value between the two pixels before color-number reduction processing is within a first standard range, the reduction function executes first color-number reduction processing by executing the processing of setting color values of the two pixels to be the same,
• wherein when a data size of the compressed image data after the first color-number reduction processing is larger than a threshold value, and when the difference value between the two pixels before color-number reduction processing is within a second standard range, which is wider than the first standard range, the reduction function executes second color-number reduction processing by executing the processing of setting color values of the two pixels to be the same, and
• wherein the processing target image data used in the second color-number reduction processing is the processing target image data before the first color-number reduction processing.